1. Field of the Invention
The present invention relates to a method for detecting hybridized nucleic acid with improved sensitivity, more specifically, to a method for improving detection sensitivity for hybridized nucleic acid which is immobilized on a solid support of sensing device for genetic analysis, by removing non-hybridized nucleic acid probe from the solid support with the aid of nuclease.
2. Background of the Invention
Hyperlipidemia is a crucial factor which causes cardiovascular disease with a high death rate. At present, most substances developed as therapeutic agents for hyperlipidemia are inhibitors of HMG-CoA reductase, one of the cholesterol biosynthetic enzymes, and are useful for the treatment of hypercholesterolemia and hyperlipidemia.
The probe-based assay is useful to detect, quantify and analyze nucleic acid. Nucleic acid probe has long been used for sample analysis of bacteria, fungi, virus or other organisms to examine the existence of target nucleic acid (see: U.S. Pat. No. 4,851,330; U.S. Pat. No. 5,288,611; U.S. Pat. No. 5,567,587; U.S. Pat. No. 5,601,984; U.S. Pat. No. 5,612,183). In addition, the probe-based assay is useful to diagnose genetic diseases. Nevertheless, the probe-based assay has not been commercially applied in the art, since it does not meet the requirements of specificity, sensitivity and reliability.
Recently, the sequence analysis of human genome has motivated the development of DNA chip to analyze genome sequence and diagnose disease. DNA chip is prepared by immobilizing single strand DNA probes with previously known sequences on a solid support such as silicon or glass with a high density. Upon reacting unknown sample onto the chip, hybridization occurs between the probe on the chip and its complementary DNA in the unknown sample. By detecting the said hybridization on the chip, nucleic acid sequence in the unknown sample can be determined.
By employing the DNA chip, enormous genetic information can be analyzed in a simple and simultaneous manner and the relationship between the genes can be elucidated, which allows broad applications of the DNA chip in the field of diagnosis of hereditary disease and cancer, investigation of mutants, detection of pathogenic microorganism, analysis of gene expression and development of new drug. In addition, the chip is applicable to almost all bio-industry such as mass production of the antidote by screening genes coding for detoxification material using it as a sensor of microbial or environmental pollution, plant production for medical use and low-fat meat. And then, it can bring about a revolutionary development in the bio-industry.
DNA chip can be classified into two groups, i.e., oligo chip and cDNA chip depending on the kinds of probes thereon, and into photolithography chip, pin-type spotting chip, inkjet-type spotting chip and electronic addressing DNA chip depending on chip fabrication method. Nevertheless, all of the DNA chips until now share common points that various single strand DNA probes are immobilized on a chip and desired information can be obtained by measuring the hybridization degree of target DNA.
Therefore, in order to obtain precise results, it is very important to develop a detection method assuring accurate hybridization signal of the target DNA and the probe DNA on a chip.
The conventional DNA chips detect residual signal on the surface of chip by a confocal microscope or a CCD camera after labeling fluorescence to target DNA and reacting it to the probe on the chip (see: U.S. Pat. No. 6,141,096). In the fluorescent image analysis, various researches for increasing the quantity of fluorescence that can be attached to the chip surface have been made to detect the signal using relatively low-priced CCD-type scanner instead of the conventional expensive confocal-type scanner. For example, many approaches are being performed such as a method employing 3-dimensional hydrogel pad (see: Anal. Biochem., 259:34-41, 1998), a method for increasing density of the probe and fluorescence using dendrimer (see: U.S. Pat. No. 6,117,631) and a method for immobilizing probe onto the porous surface using glass support with a specific pore form (see: Microarray Biochip Technology, pp. 87-117, Edited by Mark Schena, 2000 Bio Techniques Books, Natick, Mass., U.S.A.).
However, the prior optical detection methods have been proven to be less satisfactory in a sense that it is very difficult to detect small quantity of the hybridization signal, to detect the signal accurately due to background noise, to miniaturize and to gain digitalized output. In order to overcome the said shortcomings, many approaches to develop a new detection method in which results are obtained in a form of electric signal instead of optical signal are being performed.
A method for detecting DNA hybridization by the aid of electrochemical technique using conductive metal compound has been reported in the art (see: U.S. Pat. No. 6,096,273; U.S. Pat. No. 6,090,933), where DNA hybridization is detected electrochemically by measuring the redox marker of conductive metal complex upon DNA hybridization (see: Anal. Chem., 70:4670-4677, 1998; J. Am. Chem. Soc., 119:9861-9870, 1997; Analytica Chimica Acta, 286:219-224, 1994; Bioconjugate Chem., 8:906-913, 1997).
In addition, the researches for analyzing the hybridization without fluorescence or any other tagging substance are in active progress. For example, there is a method for measuring the binding capacity between the DNA oligomer probe and target DNA using microfabricated cantilever, by which, a single nucleotide difference can be analyzed (see: Science, 288:316-318, 2000). Besides, a method for measuring the mass difference caused by DNA hybridization using quartz crystal microbalance (see: Anal. Chem., 70:1288-1296, 1998) or using MALDI mass spectrometry (Matrix Assisted Laser Desorption/Ionization mass spectrometry) is under development as well (see: Anal. Chem., 69:4540-4546, 1997; U.S. Pat. No. 6,043,031).
As illustrated above, in all detection methods employed in DNA chips using complementary DNA binding, the detection selectivity for hybridized DNA can be improved by maximizing the difference of pre-hybridization signal and post-hybridization signal. Accordingly, it is expected that decreasing or removing background signals caused by non-hybridized single strand probe or signals caused by non-specific binding of target nucleic acid to probe may improve the detection accuracy and selectivity for the hybridized DNA.